Monojet+MET events with large width

Asked by Saurabh Niyogi on 2018-06-20

Hello,

I am generating monojet+MET events using the syntax p p > j vl vl where I have an s-channel propagator which can have large width (0.1M, say). I am using default bwcutoff (i.e. 15).

 How does MG5 compute cross section in such case ? I guess it gives both on and off shell contributions with this syntax. If I use a smaller bwcutoff (say 5), then does it give more contribution near the resonant mass for a large width resonance ?

If it uses narrow-width approximation for the calculation, then the output is reliable for how large a width (i.e. where one should stop in terms of width for reliable number)?

Btw, I am not worried about any interference.

Thanks in advance.
Saurabh Niyogi

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2018-06-20
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2018-06-20

Hi,

> How does MG5 compute cross section in such case ? I guess it gives both on and off shell contributions with this syntax.

Correct.

> If I use a smaller bwcutoff (say 5), then does it give more contribution near the resonant mass for a large width resonance ?

The event distribution is in this case NOT impacted by the value of bwcutoff.
The only difference between 5 and 15 is in the first case the absence (for some events) of the
s-channel particle in the event record. So the only impact is how the lhe event file is written (the format not the physical content)

Actually the presence/absence of such s-channel particle in the event record is used by parton shower to have a specific handling of s-channel particle. and therefore changing such parameter can have some impact with the parton shower.

However in the case of large width (width >= 10% of the mass --not fully sure of the exact value here)
Such s-channel particles are NEVER written in the events files since this does not make any sense for the parton-shower to preserve such invariant mass.

> If it uses narrow-width approximation for the calculation, then the output is reliable for how large a width (i.e. where one should stop in terms of width for reliable number)?

Well the naive estimator of such error on the total cross-section is width/Mass.
For distribution this deeply depends of the variable obviously.

Cheers,

Olivier

> On 20 Jun 2018, at 19:37, Saurabh Niyogi <email address hidden> wrote:
>
> New question #670351 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/670351
>
> Hello,
>
> I am generating monojet+MET events using the syntax p p > j vl vl where I have an s-channel propagator which can have large width (0.1M, say). I am using default bwcutoff (i.e. 15).
>
> How does MG5 compute cross section in such case ? I guess it gives both on and off shell contributions with this syntax. If I use a smaller bwcutoff (say 5), then does it give more contribution near the resonant mass for a large width resonance ?
>
> If it uses narrow-width approximation for the calculation, then the output is reliable for how large a width (i.e. where one should stop in terms of width for reliable number)?
>
> Btw, I am not worried about any interference.
>
> Thanks in advance.
> Saurabh Niyogi
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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